Abstract
A folio fragment attributed to the Fatimid period in Egypt was found to bear tufts of white crystals associated with the orange-brown and yellow paints. Raman spectroscopy identified a mixture of arsenic sulfide-based pigments in the orange-brown and yellow areas, along with vermilion in the outlines of the figures. X-ray microdiffraction, Raman spectroscopy, and energy-dispersive X-ray spectrometry identified the white crystals as hörnesite [Mg3(AsO4)2·8H2O]. Synthesis of this compound at ambient temperature and elevated relative humidity over a period of 3 years, on paper painted with arsenic sulfide pigments and treated with magnesium carbonate, suggests the possibility that a magnesium-based deacidification treatment may have contributed to the formation of this phase on the folio fragment. This work highlights the potential damage that may be sustained by arsenic sulfide-based media through exposure to deacidifying suspensions such as the ones often used in the past to treat works of art on paper and historic documents.
Highlights
The chemistry of the arsenic sulfides and, in particular, the photodegradation of realgar (α-As4S4) as found in works of art and mineral collections have been extensively studied [1,2,3,4,5,6,7,8]
Raman spectra acquired in situ from the orange-yellow and yellow pigment particles used to paint the bodies of the lion and hare gave bands at ca. 172, 187, 191, 201, 220, 234, 273, 318, 331, 344, and 361 cm−1 that are consistent with frequencies reported for mixtures of pararealgar and the orange χ-phase (Fig. 4) [5]
Other bands reported for realgar overlap those of pararealgar and/or the χ-phase, while additional bands observed at ca. 137 and 149 cm−1 cannot be firmly assigned
Summary
The chemistry of the arsenic sulfides and, in particular, the photodegradation of realgar (α-As4S4) as found in works of art and mineral collections have been extensively studied [1,2,3,4,5,6,7,8]. Experimental A preliminary examination of both sides of the folio under low-power magnification revealed tufts of white acicular crystals superimposed on and exclusively associated with striated laths of orange and yellow pigment particles suggestive of arsenic sulfides (Fig. 2).
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